Group III nitride crystals are used as a material for photonic devices, electronic devices, and other semiconductor elements. Those semiconductor elements are generally in such a form that a group III nitride film serving as a device functional layer is formed on a substrate having a group III nitride crystal of high crystal quality on its surface (hereafter referred to as a “group-III-nitride-crystal substrate”).
In view of crystal quality, manufacturing cost, and other problems, the group-III-nitride-crystal substrate is generally available in a form of what is called an epitaxial substrate in which a group III nitride crystal is epitaxially formed on a given single-crystal base material to approximately 10 μm at most (to such an extent that no warpage can occur due to a difference in thermal expansion coefficient). Typical methods for forming such a substrate include MOCVD (metal-organic chemical vapor deposition), MBE (molecular beam epitaxy), and other thin-film deposition techniques.
The technique for heating a single-crystal sapphire substrate in a N2-CO mixed gas at high temperatures to thereby simultaneously form an aluminum oxynitride layer (ALON layer) and an AlN layer on the single-crystal sapphire substrate are already well-known for formation of the group-III-nitride-crystal substrate (cf., for example, Japanese Patent Application Laid-open Nos. 2004-137142 and 2005-104829). It is, however, noted that the methods disclosed in Japanese Patent Application Laid-open Nos. 2004-137142 and 2005-104829 have allowed the formation of an AlN layer, but that both Japanese Patent Application Laid-open Nos. 2004-137142 and 2005-104829 have neither disclosed nor suggested any specific technique for achieving good surface flatness in a group III nitride crystal. Japanese Patent Application Laid-open No. 2004-137142 has merely exemplified the presence of roughness of the order of 400 nm, at most, in the sapphire substrate after formation of the AlN layer.
In order for a device functional layer to have good device characteristics, it is necessary to reduce, as much as possible, dislocations that may propagate from the group-III nitride-crystal substrate to the device functional layer. In particular, since in those devices that use an epitaxial substrate, dislocations can occur due to a difference in lattice constant (due to lattice mismatch) at the interface between a base material and a group III nitride crystal, thread through a group III nitride film which is a device functional layer, and mostly propagate into the surface, then it is necessary to prevent this from happening. Reducing those dislocations can allow for, for example, an improvement in luminous efficiency for light-emitting devices, and a reduction in dark current for light-receiving devices. Further for electronic devices, an improvement in mobility can be allowed for.
The inventor of the present invention has found out that high-temperature heat treatment of an epitaxial substrate could improve the substrate-surface flatness and that such heat treatment could reduce dislocations in a group III nitride crystal, which fact is now already well known (cf., for example, the specification of European Patent Publication No. 1614775).
At the time of creating the invention relating to the method for improving the crystal surface flatness of a group III nitride crystal disclosed in the specification of European Patent Publication No. 1614775, the inventor of the present invention had made the inference that in achieving the effect of reducing dislocations in addition to improving the surface flatness of an epitaxial substrate, since the epitaxial substrate were going to undergo heat treatment for achieving a thermal equilibrium state, longer heat treatment time would be desirable.
The inventor of the present invention had also considered at the same time that, since the method for improving flatness disclosed in the specification of European Patent Publication No. 1614775 is to take advantage of the regularity of crystalline arrangement of a single-crystal base material, thereby improving the crystal quality of a group III nitride crystal which is an upper layer formed on the base material, the base material would preferably be made of a material that would neither decompose nor melt in the temperature zone for heat treatment or that would not react strongly with a group III nitride crystal that forms an upper layer. This was because of the necessity of avoiding the occurrence of disorder in the crystalline arrangement of the base material during heat treatment.
It had also been considered preferable that the base material and the upper layer would produce no remarkable reaction product thereof at their interface during heat treatment, and more specifically either that there be no presence of reaction products at their interface after heat treatment, or that even in the presence of a reaction product, the thickness of the reaction product be not more than one tenth of the thickness of the upper layer. This is because the thickness of a reaction product above this range could result in a risk of degradation of the surface flatness of the upper layer.
The inventor of the present invention had thus concluded that those cases, where heat treatment had produced an ultra-thin reaction product across or partly across the interface between the base material and the upper layer, were not exclusively excluded from the invention relating to the method of improving crystal surface flatness disclosed in the specification of European Patent Publication No. 1614775.